(1) Field of the Invention
This invention relates generally to a hydrometallurgical process for the production of precious metals, especially silver, from precious metal containing solid materials, especially refractory silver ores. As used herein the term "precious metal(s)" means one or more from the group consisting of silver, gold, and mixtures of silver and gold. In particular, this invention is concerned with the processing of ores containing additionally carbonates and oxidic minerals (also known to be cyanicides) which are not economically treatable by conventional methods. This invention is also concerned with the processing of ores in a cyclic process which avoids substantial discharge of process materials to the environment. In accordance with this invention, silver or gold is produced after processing the silver or gold bearing ore with solutions containing chloride and hypochlorite ions. The chloride/hypochlorite solution may be regenerated electrochemically and reused in the process, rather than being discharged.
(2) Prior Art
Cyanidation is currently the most widely used technique for recovery of silver and other precious metals from ores. However, many precious metal bearing ores contain significant quantities of materials called cyanicides, and therefore are not economically treatable by current techniques. The process of this invention is applicable to cyanicide containing silver ores. The process of this invention also avoids the toxicity problems inherent in the cyanidation technique.
The prior art teaches use of hypochlorous acid, of hypochlorite ion, as a source of generating chlorine gas, or as a pretreatment followed by conventional cyanidation techniques. The prior art does not teach or suggest use of a large volume of a solution containing both chloride ion and hypochlorite ion, under conditions not producing chlorine gas, as a lixiviant for precious metal containing solid materials. In fact the art appears to teach away from use of chloride/hypochlorite solutions. For example, Fink and Putnam, U.S. Pat. No. 2,283,198, indicates that solutions of chlorine are ineffective with ores containing calcite. Hendrickson and Dagson, U.S. Pat. No. 3,594,157, teaches precipitation of silver values with hypochlorite solutions, rather than solubilization of the silver thereby. Scheiner, et al., U.S. Pat. No. 3,639,925, employs hypochlorite solutions as a pretreatment, but must use conventional cyanidation techniques for efficient gold recovery. Scheiner, et al., U.S. Bureau of Mines Report of Investigations No. 7736, indicates that electrolytically generated hypochlorite ion in the presence of chloride ion produces only 34% silver extraction after extensive treatment. The cumulative teaching of the prior art appears to suggest that use of chloride/hypochlorite solution would not lead to an efficient process for hydrometallurgical extraction of silver values from their ores.
At the present time in the industry, hydrometallurgy generally employs cyanidation techniques, which generate extremely toxic wastes, which could constitute an extreme environmental and safety hazard. Moreover, the cyanidation technique is not applicable to ores containing significant quantities of calcium carbonate, such as calcite, and other cyanicitic minerals.